3120 chainsaw problem "Need Help"

underwaterlogger · January 02, 2016, 12:05:03 PM

I use a 3120 for milling. It cranks right up and is ready to go. But after cutting a couple slabs it's a pain to keep it running. Then when I do get it running I have to give it the gas it won't idel. Anyone have this problem before?

mad murdock · January 02, 2016, 04:25:44 PM

I would check the spark plug, and plug lead, take all the plastic off and give it a thorough cleaning, so it does not build up extra heat while running. The fine milling dust can accumulate in all the wrong places leading to poor cooling air flow. take off the starter cover and make sure there is not build up on the flywheel and surrounding area as well. After that, I would get a carb kit for it, and install that, and don't forget to check or change the fuel filter. These PM items don't cost a lot, and make a big difference in the saws performance day to day.

starmac · January 02, 2016, 10:00:32 PM

How sdoes the tank vent on the 3120?

underwaterlogger · January 03, 2016, 03:12:00 AM

Thanks a lot mad Murdock I believe the carb is fine it cranks so well and runs great til I cut a few slabs. Is this from overheating. If so what causes it to act crazy when over heated. Tank vent? Will this be a tube?

old2stroke · January 03, 2016, 03:53:22 PM

Sounds like a restrictive fuel tank vent. As fuel is sucked out of the tank, the vent valve lets air in to replace the fuel, if the valve is completely blocked you should run into fuel starvation problems after less than 10 minutes of full throttle operation so it is probably only partially blocked. The most common valve is a short piece of tubing inserted through the wall of the tank. It's open on the outside end and squeezed shut on the inside end. Called a "duck bill valve" and they are prone to being destroyed by ethanol in the fuel. Another common vent is a one-way valve built into the fuel tank cap. An easy way to determine if this is the problem is just to loosen off the cap and let air in whenever your problem shows up. Good luck, the little beggars can be hard to find.

underwaterlogger · January 03, 2016, 04:53:48 PM

The problem will happen even right after I'm done fueling so when I recrank it it doesn't wanna run unless I give it fuel. Keep in mind it only does it after a few cuts are made.

underwaterlogger · January 03, 2016, 04:54:15 PM

Oh yea and the motor fins are clean as new.

starmac · January 03, 2016, 05:49:43 PM

Could it be an air leak somewhere causing it to lean out once warmed, if so that would not be a good deal.
Not a big two stroke mechanic, but I think the ignition could also be a possibility, mabe breaking down some as things heat up. This would be easy check by checking for a difference color spark between cold and heated to the point it isstarts giving you trouble.

ehp · January 03, 2016, 06:22:17 PM

sounds like its going very rich on the low speed jet side , if you got a dealer or someone that knows what there doing have them adjust the carb or at least make sure the carb is set correct , only 1 jet on it unless its one of the first 3120's made , also if your air filter is plugged up bad that will cause the saw to go rich as well

underwaterlogger · January 04, 2016, 03:30:32 AM

I cleaned the crap out of the air filter. I'm going to see if that makes a difference. That's for all the help all you guys are so helpful.

old2stroke · January 04, 2016, 04:11:52 PM

How long does it operate at full throttle before the problem shows up? Not sure how long it takes to cut a few slabs.
Does the saw have to be shut off and let cool down before it will start and run normal for awhile and does that saw have electronic ignition or the old points setup? Some semiconductor ignition modules are prone to shut down when they get too hot.
When it does its trick, pull the spark plug and let us know what colour it is and if it is wet or dry.

CTYank · January 07, 2016, 10:17:55 PM

Even easier for checking the spark- an inline tester. Lisle makes a decent one. connect one end to the plug, the other to the plug lead. There's a transparent chamber in the middle with a spark gap.

After milling those few slabs, shut it off and connect the tester. Fire it up and look at the spark. You might want to hook it up before milling, to see what "baseline" spark looks like, so you could spot differences.

grweldon · January 08, 2016, 11:33:19 AM

The crankcase on 2-cycle engines need to stay well sealed. Depending on how old the saw is (I assume it's quite old) and assuming that they have never been replaced, crank seals will wear out and this will be accelerated by ethanol in the fuel. I use only non-ethanol fuel in all my small engines because I can get it.

As the saw gets warm and up to operating temperatures, everything expands (including seals) and the oil film that may have been hiding the leaking seals (may not be hiding them at all, but it's possible), the air entering the crankcase decreases the efficiency of the fuel transfer to the top of the piston and can possibly affect the mixture ratio as well, causing leaning of the mixture and subsequent poor running and hard starts.

I'm not saying this is your problem, but it could be, or at least one problem contributing. I fly a 2-cycle engine on my ultralight aircraft and I do all my own engine maintenance. I'm quite familiar with the important things of the conventional-style 2-strokes. I bet my life on it when I fly. I inspect one crank seal on every pre-flight inspection because I can see it. Not so easy on a chainsaw.

If this saw is old and has never been rebuild and had crank seals replaced, it may be the time. If it's leaning out when you use it, it won't last long before it's trash. Oh, and the suggestion above about checking the ignition module after heating is a good suggestion as well...

Grandedog · January 08, 2016, 01:24:07 PM

Howdy,
3120's have solid intake manifolds and will vapor lock with enough heat. If you're going to use the 3120 for milling, you should have the milling jet in the carb. It's a little larger than the stock jet and offers more fuel to stay cooler. Also, when milling with a 3120 you should use as high of octane fuel that you can get. The octane will help cool the saw. If you are having heating and pre detonation issues it will show as pitting on the top of the piston on the exhaust side.
Regards
Gregg

AdkStihl · January 09, 2016, 08:35:08 PM

Quote from: ehp on January 03, 2016, 06:22:17 PM
sounds like its going very rich on the low speed jet side , if you got a dealer or someone that knows what there doing have them adjust the carb or at least make sure the carb is set correct , only 1 jet on it unless its one of the first 3120's made , also if your air filter is plugged up bad that will cause the saw to go rich as well

How you been Ed?